The principles of the formation of double-lock-stitches by sewing machines having a stitch plate through which the needle is thrust from the arm or head of the sewing machine to carry a loop of a needle-thread to the underside of the fabric workpiece or workpieces advanced along the stitch plate below which a bobbin housing for the gripper thread is provided, are well known in the art. Mention may be made of U.S. Pat. Nos. 4,074,645 and 4,137,858 which disclose rotary gripper arrangements, to U.S. patent application Ser. No. 005,781 filed 23 Jan. 1979 in the same field, to the art of record in the files of said patents and application, the German Pat. Nos. 422,546 and 739,559, and to German patent document No. 2741,830 in this connection.
In general, as a workpiece, usually consisting of two or more superposed fabric layers, is advanced across the stitch plate past the sewing head or its arm overhanging the stitching location, the double-lock-stitch is formed by the successive passes of the needle, which carry the needle-thread fed from above the workpiece, to a location below the workpiece at which the needle-thread loop is formed.
Below the workpiece, the needle-thread loop is engaged with a gripper thread fed from a bobbin which is received in a bobbin housing located below the stitch plate.
The present invention is concerned with a system of this type using a rotary gripper, preferably a gripper rotatable about a horizontal axis although the principles hereof are applicable to grippers rotatable about vertical axes or inclined axes as well.
It has long been recognized that the uncontrolled movement of the needle-thread loop below the stitching plate can adversely effect the uniformity of the stitch seam either on the underside alone or on both surfaces at which the stitching may be revealed.
In German Pat. No. 422,546, there is described an arrangement for effecting the movement of the needle-thread loop which consists of a leaf spring fixed to the bobbin-housing mount and which cooperates with a nose or project on the bobbin housing to retard and control the movement of the needle-thread loop during withdrawal of the needle through the hole in the stitch plate and through the fabric workpiece.
In other words the needle-thread loop is retained and briefly clamped with a stressing of the spring. Upon release of the clamping action during withdrawal of the thread, the leaf spring is impelled against the tighter loop drawn along the underside of the fabric. This can eliminate, for the most part, the formation of knots and bunches along the underside of the seam which can result when the movement of the needle-thread loop is uncontrolled. Bunching and knotting in this manner is especially characteristic of the usual highly twisted threads.
A similarly operating spring is described in German patent document No. 27 41 830. In this arrangement, a spring rod stressed at one end functions as a counterpoint for the gripper to receive the needle-thread loop cast off from the gripper point to retard movement of the gripper.
Both of these arrangements have the advantage that they allow the needle-thread loop to be drawn to a relatively small diameter as the stitch formation progresses in a controlled manner, thereby limiting the space or slackness which can form the knots or bunching. In fact, these systems have proven to be highly effective in normal double-lock stitch seaming operations in which the gripper thread and needle-thread loops hook together ultimately at approximately the center of the thickness of the fabric.
When however the needle-thread tension is, relative to the gripper thread tension, somewhat weaker, the more greatly tensioned gripper thread loops around the needle thread and limits the spring of the retaining member which impels this needle-thread loop against the underside of the fabric. This can occur with so-called raised seams when embroidery stitches are to be formed and the embroidered pattern must lie above the surface of the fabric. Similar difficulties are encountered in the sewing of buttonholes for example as described in German Pat. No. 739,559. The earlier spring devices are then ineffective.